An ad-hoc wireless network system is an autonomous communication system that allows wireless mobile devices to communicate directly with each other without going through a base station, or to communicate with each other via another mobile device between them. Even if the system currently has a configuration in which only some of the mobile devices can reach each other, a data item transmitted by a source device is relayed by other mobile devices to the destination device.
There are two types of ad-hoc wireless networks, i.e., an ad-hoc network consisting of only wireless LAN devices such as laptop computers as illustrated in FIG. 1, and a mesh network consisting of access points and wireless LAN devices as illustrated in FIG. 2. In this context, both types of network are referred to as an “ad-hoc wireless network”.
In an ad-hoc wireless network, data transmission is performed directly between mobile devices as long as they are located within communication range. In FIG. 1, direct data transmission is performed between devices 1 and 2, devices 2 and 3, and devices 3 and 4. Between out-of-range devices, such as devices 1 and 4, devices 1 and 3, and devices 2 and 4, data transmission is performed via an in-between device.
Each of the communication nodes in an ad-hoc wireless network (wireless LAN devices 1-4 in FIG. 1 and access points AP1 through AP4 in FIG. 2) continually transmits a common broadcast signal (hereinafter referred to as a “beacon signal”) at constant intervals to communicate the existence of that node to the nearby communication nodes or the wireless LAN devices currently belonging to that node in order to enhance connection with the adjacent nodes or the wireless LAN devices.
In general, beacon transmission interval is determined by the initial node that sets up the network, and the lowest transmission rate is generally selected for the purpose of widening the communication range as much as possible. Selecting a low transmission rate means that the channel time occupied by the beacon signal becomes longer. This leads to a serious problem of consumption of wireless resources when the number of nodes transmitting the beacon signals increases.
FIG. 3 is a schematic diagram for explaining the issue of channel occupation that becomes conspicuous as the number of communication nodes increases in an ad-hoc wireless network. For example, mobile device 1 in FIG. 1 sets up a network as the initial node, and is transmitting a beacon signal B1 at prescribed intervals. Then mobile device 2 participates in the network as a new node, and starts transmitting a beacon signal B2 at the same intervals. In this case, the wireless channel is occupied by beacon signals B1 and B2, and therefore, the band occupancy ratio doubles.
Another problem is collision between beacon signals and data packets transmitted from the communication nodes. With the wireless LAN IEEE 802.11 standard, packet collision with a beacon signal is avoided stochastically by a collision avoidance mechanism. However, this mechanism works on the assumption that packets are retransmitted from each node after a random time, and packets collide with each other only if the random times agree with each other between two or more nodes. In addition, the more the number of nodes, the higher is the collision probability.
The ANSI/IEEE 802.11 standard states that a beacon signal is always transmitted at constant intervals. See, ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and Physical layer (PHY) specifications, 1999. This specification accepts a transmission delay under congestion, but does not provide a change in beacon interval. Consequently, a beacon signal is always transmitted at constant intervals even if a large part of the wireless channel is occupied by beacon signals due to many nodes existing in communication range. As a result, the situation illustrated in FIG. 3 occurs.
A system designed so as to allow a wireless LAN device to connect itself to a less congested access point among multiple wireless LAN access points by measuring a transmission time delay under congestion of beacon signals is proposed. See, for example, JP 2003-60657A. However, again the beacon interval is kept constant.
Another proposal is that multiple mobile devices currently connected to an access point change the beacon receiving interval by linking up the operating modes with each other. See, for example, JP 2004-128949A. In this publication, each mobile terminal connected to an access point changes and adjusts the beacon receiving interval n times the reference interval; however, the beacon transmission interval at the access point is kept constant.
It is also proposed, in data transmission between a base station and wireless cellular devices belonging to this base station, to generate a random number at each cellular device to distribute the transmission interval in order to avoid data collision. See, for example, JP 7-298357A.